1. Field of the Invention
The present invention relates to a permanent magnet type motor and an X-ray computed tomography (hereinafter, referred to as X-ray CT) apparatus.
2. Description of the Related Art
Conventionally, it has been well known that an example of X-ray CT apparatus is configure as shown in FIG. 11 which is a partial sectional view showing essential portions thereof. The conventional X-ray CT apparatus comprises: a stationary tubular frame 51 having a small diameter portion and a large diameter portion to form a substantially L shaped cross section with respect to a tube axis O; a bearing 52 arranged at the small diameter portion of the frame 51; a rotational tubular frame 53 having a small diameter portion rotationally connected to a movable side of the bearing 52 and a larger diameter portion to form a substantially L shaped cross section with respect to the tube axis O; a ring shaped rotor 54 fixed at the large diameter portion of the rotational tubular frame 53; a stator 57 having a stationary iron core 55 fixed at the inner periphery side of the larger diameter portion of the frame 51 to face the rotor 54 via a predetermined gap therebetween and a stator winding 56 wound on the core 55; and a slip ring 58 fixed on the inner side of the large diameter portion of the rotational tubular frame 53 and adopted to supply power to an X-ray tube (not shown) fixed on the rotational tubular frame 53.
In addition, a gear 59 made of a magnetic material is fixed at an end part of the large diameter portion of the rotational tubular frame 53. A magnetic sensor 60 is fixed on the end of the stationary tubular frame 51 to be proximal to the gear 59. With these magnetic sensor 60 and gear 59, a rotational position of the rotor 54, i.e., a rotational position of the X-ray tube is detected. In this case, when a tooth of the gear 59 or a protrusive portion thereof is opposed to the magnetic sensor 60, the magnetic flux density crossing the sensor 60 increases and the resistance value thereof increases. In contrast, when a tooth groove or a recess portion of the gear 59 is opposed to the magnetic sensor 60, the magnetic flux density decreases, and the resistance value decreases. Thus, the rotational position of the X-ray tube is detected.
Further, a plurality of pairs of permanent magnets 61 each configuring a magnetic pole are fixed at the outer periphery portions of the rotor 54. A Hall effect device or a Hall effect IC utilizing a Hall effect, although not shown, is disposed on the stationary tubular frame 51 by means of a support member (not shown) in proximity to the side surface of the permanent magnets 61. By this Hall effect IC, a magnetic pole position of the permanent magnet 61 can be detected.
The rotational tubular frame 53 supports an X-ray detector for detecting an X-ray emitted from the X-ray tube and transmitted through a detection target, in addition to the X-ray tube. In addition, is mounted on the stationary tubular frame 51 a computer section which reconstructs tomographic image data of the detection target based on an output of the X-ray detector and a rotational position of the X-ray tube on the rotational tubular frame 53. A signal transmission system storage section 62 is designed so as to transmit X-ray data detected by the X-ray detector to the computer section on the stationary tubular frame 51 which is included in an X-ray related device storage section 63 via an optical communication line. In FIG. 11, the X-ray related device storage section 63 comprises the above-described X-ray rube, X-ray detector, computer section, and a power source for the X-ray tube. In addition, a substantially tapered internal sheath cover 26 is arranged at the inner periphery side of the rotational tubular frame 53.
In the above-described conventional X-ray CT apparatus, the rotor 54 for rotating the rotational frame 53 is disposed and fixed at the outer surface side of the large diameter portion of the rotational frame 53 which is distant from the bearing 52 or the center axis O. In addition, the stator 57 is positioned at the outer periphery side of the rotor 54 and is fixed at the inner periphery surface of the stationary tubular frame 51 via a predetermined gap from the rotor 54. Further, the slip ring 58, positioned on the large diameter portion of the rotational tubular frame 53 and adopted to supply power to the X-ray tube, is disposed and fixed at the inner periphery side of the rotational frame 53 at a position at which the rotor 54 is disposed. Thus, there has been a problem that the whole X-ray CT apparatus is large sized.
Moreover, the fact that the whole X-ray CT apparatus is large sized causes a problem with respect to the cost reduction of the X-ray CT apparatus.
On the other hand, as an example of the electric motor used for the conventional X-ray CT apparatus, an electric motor provided with a magnet type encoder is used. FIG. 12 shows a part of this type of motor having a magnetic gear 59 which is composed as a piece other than a motor section 64. The gear 59 is provided as a detection body of the magnet type encoder. The gear 59 is made of a thick magnetic material by means of cutting processing.
In addition, a resolver (not shown) is mainly used as a rotation position detector of a large diameter motor of hollow shaft type without providing a motor shaft, such as that shown in FIG. 11.
Like a motor, the resolver is composed of a stator and a rotor. Two pairs of windings shifted by 90 degrees each other are disposed at the stator and rotor, respectively. When the stator is energized as a primary side, an output voltage signal fluctuates in response to a rotational position of the rotor acting as a secondary part. A rotational position of the rotor is detected by means of this output voltage signal.
In this way, a detection target of the magnet type encoder is provided other than the electric motor body, and thus, has a defect that the target requires a mount space and parts and assembly cost.
Further, in the large diameter motor of hollow shaft type, a resolver must be used. The resolver enables detection of an absolute position and has high precision. However, the resolver is high in cost from a structural aspect.